This invention relates to wavelength filtering in optical waveguide systems. More particularly, it concerns the separation of narrow wavelength bands from a broader spectral region through use of a fiber optics device distinct from the device carrying wavelengths of the broader spectral region.
In commonly owned U.S. Pat. No. 4,342,499 issued Aug. 3, 1982 to the present inventor, tuned, dispersive lateral coupling between two fiber cores has been described. However, the construction of a filter by such means having a bandwidth and less than 0.001 of the central wavelength has proven to be difficult principally due to variation in fiber diameter out of limitations of the drawing process for the fiber device. Although the art of fiber optics drawing is improving rapidly in the attainment of accurate fiber physical characteristics, there is a present need for a filter capable of passing bandwidths of less than one Angstrom in the one to two micron range of the optical spectrum.
Narrow band filtering of the type referred to is useful in a large communications system since the cost of trunk lines is reduced as the number of signal channels per fiber is increased. Still further, the number of possible ports in a switching station goes up as the number of channels per fiber increases. The latter of the foregoing uses for narrow band filter is emphasized by the need for switching to be categorized broadly into two techniques. Commonly, switching requires the crossing of m lines over n in a rectangular matrix and providing switches at each cross-over point. Such a system relies primarily on a very large number of two-position switches.
A second manner of switching utilizes the broadcast by m stations where each broadcast station uses one of many possible "channels". Switching is accomplished by tuning the proper receiving port to the same channel as the selected sending port.
In terms of broadcast switching, a multi-channel transmission line offers an improvement over free space broadcasting since the energy of any channel need not be as great. Channel energy is routed along a common line and remains on that line until it passes a port at which energy is removed. Optical fiber waveguide systems offer an improvement over existing channel broadcast switching stations because of the very large number of channels that can be carried by a single fiber or waveguide line.
If a single mode optical fiber waveguide carries 100 channels spread over 1/10th of the optical spectrum, it will be loaded in terms of bandwidth only when the modulation rate on each channel is around 5.times.10.sup.10 signal pulses or bits of information per second (assuming the wavelength region to be around 1 micron). However, such high modulation rates are difficult to achieve with present technology. Further, the signal pulses are diffused relatively quickly as they travel along the length of an optical fiber because of group velocity dispersion. Still further, there are few single sources of information with repetition rates higher than video rate. Therefore, to fully load such a 100 wavelength channel fiber, it would be necessary to multiplex in the time domain approximately 10,000 video signals. Apparatus for time domain multiplexing is relatively very expensive and with the present state of the electronics art, it is not possible to do time division multiplexing in packets with video rate signals and trunk rates of 5.times.10.sup.10 pulses per second. It is barely possible, if at all, at 5.times.10.sup.9 pulses per second.
For the above reasons, it would be extremely advantageous to be able to use 10,000 to 100,000 wavelength channels on a single fiber and not require use of time domain multiplexing. To accomplish this, one needs to filter a line width of 1/100,000 to 1/1,000,000 of the base wavelength, which in the one micron wavelength region is in the range of 0.1 Angstrom to 0.01 Angstrom. While the attainment of such narrow filtered line widths in a communication system would be highly desirable, a very useful system would result using a filter with 1 Angstrom line width as opposed to 10 Angstrom width.
There are various applications for wavelength filters, but for operation in the type of system referred to above for illustration, the following requirements must be met:
1. A narrow spectral line must be separated off onto a separate path. PA1 2. The remainder of the wavelength channels must be disturbed as little as possible. For example, if one were to remove one-half the energy in the remainder of the channels, the number of operable receiving ports decreases significantly.
With respect to the second point, it is not necessary to put all n receivers on a single line. The same set of signal bearing channels can be sent along several lines in parallel so that all the receivers do not have to be on the same line. However, there is an advantage in perturbing the unfiltered channels as little as possible and using the fewest number of parallel lines in a switching station.
Another area of use is in a fiber optics distribution system. In such a system, a local switching station addresses a particular receiver on a common distribution line by placing the signal on the wavelength channel to which the receiver is tuned. That is, the receiver has a fixed-tuned line tap which removes one wavelength channel as it passes on the common line. Again, it is desirable to leave the untapped channels as unperturbed as possible. The advantage of increasing the number of channels from 1 to 10 on a local distribution line is enormous and while further increases to 100 or 1,000 channels are decreasingly advantageous they still have merit. Overcoming the problem of devising such a line tap is however a significant accomplishment.
In the existing technology, there are other approaches to narrow line filtering. For instance, the usual choice of optical line filtering means, such as prisms, diffraction gratings, Fabry-Perot interferometers, and the like are available. They all suffer from their geometry. In none of these devices is it easy to get the filtered light onto one fiber and to get the remainder of the light onto another fiber. The Fabry-Perot is the only one named which achieves line widths as low as 1 Angstrom in a small volume.
Resonant cavities are well known in longer wavelength regions of the electromagnetic spectrum, but are an exception within the optical region of that spectrum. Even where resonant cavities are used in the optical part of the spectrum, they suffer from high loss rates per cycle and are not small enough to operate in the 1 Angstrom to 0.01 Angstrom line width range.
Difficulties can be expected in using the fiber device of the invention in the form of a linear resonant cavity having highly reflective metal coatings at the ends, and in attempting single mode operation with butt-coupling to the transmission fibers. The fundamental difficulty is shared with the Fabry-Perot device. That is, the nature of a highly reflective metal surface is such that most of the light which is not reflected will be absorbed. This is inherent in the reflection process for metallic mirrors, and results in a very low efficiency through the butt-coupling at a resonant wavelength. In the case of a Fabry-Perot, this can be avoided by using highly reflective multi-layer dielectric coatings. Such multi-layer mirrors are of necessity not thin compared to the wavelength of light to be reflected. Therefore, they will not be highly efficient on the end faces of a linear fiber cavity resonator, and constitute an undesirable gap in the optical path.
A second difficulty is that the unfiltered remaining light beam is directed back along the same fiber bringing the light to the cavity.
While the above difficulties are alleviated in loop cavities of the invention, a third difficulty is common to all simple resonant cavity devices of this sort. Namely, there are many resonant lines, more or less equally spaced. The condition for resonance is that all of the phase changes resulting from factors which influence the phase of the light during one complete round trip must add up to an integral number of wavelengths. Obviously, if for a given resonant wavelengths, the optical path is 1,000 wavelengths, when there will be another resonant wavelength such that the optical path is 999 wavelength.
This will occur at a change in wavelength of about 1/1,000 of the original wavelength. Instead of filtering off one wavelength, any simple resonant cavity filters off a series of wavelengths.
The ratio of line width (of each resonant wavelength) to spacing between lines (the finesse of the cavity) is approximately proportional to the fraction of energy lost in the course of one round trip. This loss includes reflection losses, scattering, transmission losses, and losses through the input and output ports (assuming the input source is momentarily turned off).